A LTE-A (Long Term Evolution Advanced) system aggregates five 20 MHz bandwidths and supports 100 MHz through carrier aggregation, wherein standardization of the LTE-A system is going on. Carrier aggregation is a technology that aggregates a plurality of carriers and expands bandwidth to increase data rate.
A multi carrier is a whole frequency band a base station uses, and a component carrier is element carrier included in a multi carrier. That is, a plurality of component carriers composes a multi carrier through carrier aggregation.
Hybrid automatic repeat request (HARQ) will be described.
HARQ is a method combining forward error correction, (FEC) and automatic repeat request (ARQ), wherein the HARQ is used to improve transmission efficiency of packet data which is less sensitive to transmission delay. HARQ is used as connected to a physical layer, and improves decoding success rate by combining retransmitted data with data received previously. That is, according to HARQ, a receiver does not discard a failed packet but store it, and decodes it by combining it with a retransmitted packet.
According to HARQ, after transmitting a transmission block to a receiver, a transmitter receives an acknowledgement (ACK) or a Negative Acknowledgement (NACK). If receiving a NACK, a transmitter retransmits a transmission block.
FIG. 1 illustrates HARQ scheme according to the related art in a wireless communication system with carrier aggregation.
Illustrated in FIG. 1, a base station transmits data to a mobile station through a physical downlink control channel (PDCCH) and a physical downlink shared channel (PDSCH) of F1 and F2 carriers. Then if a base station receives a NACK or a timer of a base station is expired, a base station retransmits data through same frequency band. That is, data which is transmitted through F1 carrier in initial transmission is transmitted through F1 carrier in retransmission, and data which is transmitted through F2 carrier in initial transmission is transmitted through F2 carrier in retransmission.
However, if a carrier which is used in initial transmission is used in retransmission, it is difficult that data which is transmitted through a carrier with low quality in initial transmission is transmitted successfully in retransmission.
FIG. 2 illustrates a method of HARQ acknowledgement transmission according to the related art in a wireless communication system with carrier aggregation.
A base station transmits control information through PDCCHs of F1, F2 and F3 carriers and downlink data through PDSCHs of F1, F2 and F3 carriers to a mobile station. A mobile station, which receives downlink signal from a base station, transmits ACK/NACK for the downlink signal to a base station through an assigned F4 carrier. At this time, a mobile station transmits ACK/NACK through a physical uplink control channel (PUCCH) or a physical uplink shared channel (PUSCH).
According to another related art, a base station transmits data through one PDSCH and one PDCCH and a mobile station transmits ACK/NACK for the data received through one PDSCH and one PDCCH through one PUSCH and one PUCCH. That is, one downlink carrier is mapped to one uplink carrier.
But, because of carrier aggregation, ACK/NACK for data transmitted through a plurality of downlink carriers need to be transmitted through one uplink carrier. However, in case that one PUCCH can carry ACK/NACK for data received through one carrier, if a mobile station transmits ACK/NACK for data received through carriers whose number is more than the number of PUCCH, quality of uplink control information is depreciated or amount of control information transmitted is reduced. And, in case that ACK/NACK is transmitted through a PUSCH, quality of data transmitted through the PUSCH is depreciated.